Method for the removal of deposition on an optical element, method for the protection of an optical element, device manufacturing method, apparatus including an optical element, and lithographic apparatus

ABSTRACT

A method for the removal of a deposition on an optical element of an apparatus including the optical element includes providing an H 2  containing gas and one or more additional compounds selected from the group of hydrocarbon compounds and silane compounds in at least part of the apparatus includes producing hydrogen radicals from H 2  from the H 2  containing gas; and bringing the optical element with deposition into contact with at least part of the hydrogen radicals and removing at least part of the deposition. Further, a method for the protection of an optical element of an apparatus including the optical element includes providing a cap layer to the optical element by a deposition process; and during or after use of the apparatus, removing at least part of the cap layer from the optical element in a removal process as described above. The methods can be applied in a lithographic apparatus.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for the removal of depositionon an optical element, a method for the protection of an opticalelement, a device manufacturing method, an apparatus including anoptical element, and a lithographic apparatus.

2. Description of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that example, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.including part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude steppers, in which each target portion is irradiated by exposingan entire pattern onto the target portion at one time, and scanners, inwhich each target portion is irradiated by scanning the pattern througha radiation beam in a given direction (the “scanning” direction) whilesynchronously scanning the substrate parallel or anti-parallel to thisdirection. It is also possible to transfer the pattern from thepatterning device to the substrate by imprinting the pattern onto thesubstrate.

In a lithographic apparatus the size of features that can be imaged ontothe substrate is limited by the wavelength of the projection radiation.To produce integrated circuits with a higher density of devices, andhence higher operating speeds, it is desirable to be able to imagesmaller features. While most current lithographic projection apparatusemploy ultraviolet light generated by mercury lamps or excimer lasers,it has been proposed to use shorter wavelength radiation, e.g. of around13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray,and possible sources include, for example, laser-produced plasmasources, discharge plasma sources, or synchrotron radiation fromelectron storage rings.

The source of EUV radiation is typically a plasma source, for example alaser-produced plasma or a discharge source. A common feature of anyplasma source is the inherent production of fast ions and atoms, whichare expelled from the plasma in all directions. These particles can bedamaging to the collector and condenser mirrors which are generallymultilayer mirrors, with fragile surfaces. This surface is graduallydegraded due to the impact, or sputtering, of the particles expelledfrom the plasma and the lifetime of the mirrors is thus decreased. Thesputtering effect is particularly problematic for the collector mirror.The purpose of this mirror is to collect radiation which is emitted inall directions by the plasma source and direct it towards other mirrorsin the illumination system. The collector mirror is positioned veryclose to, and in line-of-sight with, the plasma source and thereforereceives a large flux of fast particles from the plasma. Other mirrorsin the system are generally damaged to a lesser degree by sputtering ofparticles expelled from the plasma since they may be shielded to someextent.

In order to prevent the damage of the collector mirror by debrisparticles, U.S. Patent Application Publication 2002/0051124 A1 disclosesa cap layer on the mirror surface to protect the mirror from sputteringdamage caused by fast ions and atoms expelled from a plasma source.Hydrocarbons are added to a space containing the mirror, and theyphysically or chemically adsorb to the surface of the mirror and thusform a protective layer on the surface. This surface layer is made up ofthe hydrocarbon molecules and possibly other contaminant particlespresent in the system as impurities, together with any further moleculeswhich are introduced into the system from the gas supply. When the fastions and atoms produced by the plasma hit the surface of the mirror,they contact the protective layer thereby dislodging the hydrocarbonmolecules from the cap layer, and damage to the mirror surface itself isavoided. A dynamic cap layer may be used. This is a cap layer which iscontinually sputtered away and replaced with further molecules and thusthe thickness of the layer remains substantially constant or within anacceptable range. In order to achieve this, the reflectivity of themirror and/or the background pressure of the space are monitored.

In the near future, extreme ultraviolet (EUV) sources may use tin oranother metal vapor to produce EUV radiation. This tin may leak into thelithographic apparatus, and will be deposited on mirrors in thelithographic apparatus, e.g., the mirrors of the radiation collector.The mirrors of such a radiation collector are foreseen to bemultilayered and may have a EUV reflecting top layer of ruthenium (Ru).Deposited layers of more than approximately 10 nm tin (Sn) on thereflecting Ru layer will reflect EUV radiation in the same way as bulkSn. It is envisaged that a layer of 10 nm Sn is deposited very quicklynear a Sn-based EUV source. The overall transmission of the collectorwill decrease significantly, since the reflection coefficient of tin ismuch lower than the reflection coefficient of ruthenium. The method ofU.S. Patent Application Publication 2002/0051124 A1 is not suited toremove, for example, Sn deposition from the surface of optical elementslike mirror surfaces, nor is it suited to remove, for example, Sideposition from optical elements. U.S. Patent Application Publication2002/0051124 A1 also does not address sputtering of particles and othercontaminants on optical elements. An improved method is thereforedesirable to address this problem.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide a method for theremoval of a deposition on an optical element of an apparatus includingthe optical element. It is a further aspect of the present invention toprovide a method for the protection of an optical element of anapparatus including the optical element. It is yet a further aspect ofthe present invention to provide a device manufacturing method. It isanother aspect of the present invention to provide an apparatusincluding an optical element, for example a lithographic apparatus,wherein the method of the invention can be applied.

According to an embodiment of the present invention, a method for theremoval of a deposition on an optical element of an apparatus includingthe optical element includes providing an H₂ containing gas and one ormore additional compounds selected from the group of hydrocarboncompounds and silane compounds in at least part of the apparatus;producing hydrogen radicals from H₂ from the H₂ containing gas; andbringing the optical element with deposition into contact with at leastpart of the hydrogen radicals and removing at least part of thedeposition.

In another embodiment, the deposition includes one or more elementsselected from B, C, Si, Ge and Sn. C (carbon) may, for example, be adeposition on the optical element due to the undesired presence ofhydrocarbons in the apparatus, like a lithographic apparatus, but mayalso be deliberately present as protective cap layer. Si (silicon) mayalso be deliberately present as protective cap layer, whereas Sn (tin)may be present due to a source that produces Sn. Further, Si may bepresent as deposition due to outgassing of the resist. In a furtherembodiment, at least part of the hydrogen radicals are generated from H₂from the H₂ containing gas by one or more radical formation devicesselected from a hot filament, a plasma, radiation, and a catalyst forconverting H₂ into hydrogen radicals. The hydrogen radical reacts withSn (or Sn oxides), C, Si (or Si oxides), etc., leading to volatilehydrides that can be removed by, for example, an exhaust. Sn and Si,that may have oxidized or which may be at least partially present asoxide as deposition or cap layer may be reduced to elemental Sn and Si,respectively, and may be removed as hydride or as halogenide. Therefore,in yet another embodiment, the H₂ containing gas may further include ahalogen gas. The halogens may form volatile halides, and may improve theremoval of e.g. Sn and Si deposition.

According to an embodiment of the present invention, a method for theprotection of an optical element of an apparatus including the opticalelement includes providing a cap layer to the optical element by adeposition process; and during or after use of the apparatus, removingat least part of the cap layer from the optical element in a removalprocess including providing an H₂ containing gas and one or moreadditional compounds selected from the group of hydrocarbon compoundsand silane compounds in at least part of the apparatus; producinghydrogen radicals from H₂ from the H₂ containing gas; and bringing theoptical element with the cap layer into contact with at least part ofthe hydrogen radicals and removing at least part of the cap layer.

In another embodiment, the process for providing a cap layer to theoptical element by a deposition process includes providing a startingmaterial including one or more elements selected from B, C, Si and Ge;providing from the starting materials species including one or moreelements selected from B, C, Si and Ge that are able to provide adeposition; and bringing the species that are able to generate adeposition into contact with at least part of the optical element.

In yet another embodiment, the process for providing a cap layer to theoptical element by a deposition process includes providing a gasincluding one or more elements selected from the group B, C, Si and Ge;providing from the gas species including one or more elements selectedfrom B, C, Si and Ge that are able to generate a deposition; andbringing the species that are able to generate a deposition into contactwith at least part of the optical element.

The deposition can be used as cap layer. Species that can be used toprovide a cap layer are, for example, Si and C, and also B (boron) or Ge(germanium) atoms and particles, and also intermediate products likee.g. SiH₂ may form (Si) depositions. Hence, the starting material mayinclude a gas, for example, including boron hydrides, hydrocarbons,silanes, or germanium hydrides, or combinations of two or more of them.In an embodiment of the present invention, the species that are able toprovide a deposition are provided by a hot filament, a plasma orradiation. In another embodiment, the cap layer is generated by hotfilament deposition, and in yet a further embodiment, the cap layer isgenerated by hot filament deposition of one or more elements selectedfrom B (derived from, for example, boron hydrides, such as BH₃), C(derived from, for example, a hydrocarbon, such as CH₄), Si (derivedfrom, for example, a silane, such as SiH₄) and Ge (derived from, forexample, germanium hydrides, such as GeH₄). In another embodiment,instead of chemical vapor deposition, the species that are able toprovide a deposition are provided by physical vapor deposition and asstarting materials instead of gasses, oxides or metals of, for example,Si and Ge can be applied. Elemental Si and Ge is evaporated or sputteredaway as species that can be deposited on the optical element. Hence, inan embodiment, the cap layer includes one or more elements selected fromB, C, Si and Ge.

In another embodiment of the present invention, the apparatus is alithographic apparatus. Further, in another embodiment of the presentinvention, the optical element includes a mirror or a grating, forexample a collector mirror or a multi-layer mirror. In anotherembodiment, the optical element includes a sensor or a reticle.

In yet another embodiment, the apparatus is designed to irradiate theoptical element with a radiation beam. In another embodiment, theoptical element includes a sensor and is designed to align other opticalelements or parts of the apparatus (for example the mask, the waver, themask table, the wafer table, etc.). The apparatus including the opticalelement may be designed to provide a radiation beam, such as alithographic apparatus. In yet another embodiment, the optical elementincludes a sensor that is designed to measure properties of otheroptical elements, for example the optical property of reflection of acollector mirror. In yet a further embodiment, the radiation includesEUV radiation having a wavelength selected from the range of 5-20 nm. Inyet another embodiment, the radiation includes radiation having awavelength selected from 365 nm, 248 nm, 193 nm, 157 nm, 126 nm, but theradiation may also include other wavelengths that may be used inlithographic applications.

During or after use of the optical element, the cap layer may partiallyor completely be removed. In an embodiment, the use of the apparatusincludes irradiating the optical element with a radiation beam, forexample the use of a lithographic apparatus applying EUV radiation.During use the cap layer may be deteriorated, due to, for example,deposition of Sn and/or sputtering away of a part of the cap layer, forexample by Sn particles from the source. Therefore, after use, or evenduring use, the cap layer may be at least partially removed and a freshcap layer may be generated, either on the optical element or on thepartially removed cap layer on the optical element. Hence, in anotherembodiment, during at least part of the use of the apparatus Sn depositson at least part of the cap layer or removes at least part of the caplayer. Therefore, in a further embodiment, the deposition includes atleast Sn, and in a variation thereon, the cap layer includes at leastSn. During use, both processes, deposition and removal, may occur,thereby providing a deteriorated cap layer. In a further embodiment,during at least part of the use of the apparatus Si deposits on at leastpart of the cap layer, for example due to outgassing from the resist.Hence, in another embodiment, the deposition includes at least Si, andin a variation thereon, the cap layer includes at least Si.

In another embodiment, the cap layer is removed in the removal processby etching, and in yet another embodiment, the cap layer is removed inthe removal process by bringing the cap layer into contact with hydrogenradicals. The hydrogen radicals may be generated in different ways. Inan embodiment of a method according to the present invention, at leastpart of the hydrogen radicals are generated from H₂ from the H₂containing gas by one or more radical formation devices selected from ahot filament, a plasma, radiation, and a catalyst for converting H₂ intohydrogen radicals, which dissociate H₂ to H radicals or H-atoms adsorbedto the surface of the catalyst. The catalyst may include transitionmetal based catalysts such as catalysts including Pd, Pt, Rh, Ir and Ru.The catalyst may also include a Ru layer, for example the surface of agrazing incidence mirror or of a multilayer, wherein Ru is included in atop layer. The radiation for producing radicals may include radiationsuch as EUV radiation, DUV radiation, UV radiation, for exampleradiation including radiation having a wavelength selected from 193 nm,157 nm and 126 nm, and the radiation may include radiation such aselectron beam or ionizing radiation, such that hydrogen radicals may beformed from hydrogen.

According to a further embodiment of the present invention, a method isprovided wherein at least part of the cap layer is provided ex situ fromthe apparatus including the optical element, and according to yet afurther embodiment, a method is provided wherein at least part of thecap layer is removed ex situ from the apparatus including the opticalelement.

According to a further embodiment of the present invention, a devicemanufacturing method includes providing a lithographic apparatusincluding an illumination system and a projection system, thelithographic apparatus further including an optical element; projectinga patterned beam of radiation onto a target portion of a substrate atleast partially covered by a layer of radiation sensitive material;providing a cap layer to the optical element by a deposition process;and during or after use of the apparatus, removing at least part of thecap layer from the optical element in a removal process includingproviding an H₂ containing gas and one or more additional compoundsselected from the group of hydrocarbon compounds and silane compounds inat least part of the apparatus; producing hydrogen radicals from H₂ fromthe H₂ containing gas; and bringing the optical element with the caplayer into contact with at least part of the hydrogen radicals andremoving at least part of the cap layer.

According to a still further embodiment of the present invention, anapparatus includes an optical element; an inlet configured to provide anH₂ containing gas in at least part of the apparatus; an inlet configuredto provide one or more additional compounds selected from the group ofhydrocarbon compounds and silane compounds; and a hydrogen radicalgenerator configured to generate radicals from H₂ from the H₂ containinggas.

According to a further embodiment, the apparatus includes an opticalelement; an inlet configured to provide an H₂ containing gas in at leastpart of the apparatus; an inlet configured to provide one or moreadditional compounds selected from the group of hydrocarbon compoundsand silane compounds; a hydrogen radical generator configured togenerate radicals from H₂ from the H₂ containing gas; and a depositiongenerator, wherein the deposition includes one or more elements selectedfrom B, C, Si and Ge.

According to yet a further embodiment, a hydrogen radical generatorincludes a radical formation device selected from a filament that can beheated, a plasma generator, a source of radiation, and a catalystconfigured to convert H₂ into hydrogen radicals. Ru is included in a toplayer. The source of radiation may be a source configured to produceradiation such as EUV radiation, DUV radiation, UV radiation, forexample radiation including radiation having a wavelength selected fromthe group of 193 nm, 157 nm and 126 nm, and the source of radiation mayproduce radiation such as electron beam or ionizing radiation, such thathydrogen radicals may be formed from hydrogen. In another embodiment,the source of a lithographic apparatus is used as source of radiation toproduce radicals.

According to another embodiment, the deposition generator is configuredto generate a deposition including one or more elements selected from B,C, Si and Ge, and includes a device selected from a filament that can beheated, a generator for generating a plasma and a source of radiation.

According to yet another embodiment, an apparatus includes an inletconfigured to provide a gas including one or more elements selected fromB, C, Si and Ge. The gas can be introduced such that species can beproduced, for example, by a hot filament to generate a cap layer on theoptical element. In yet a further embodiment, the apparatus furtherincludes an inlet configured to provide a starting material.

According to yet another embodiment of the present invention, alithographic apparatus includes an optical element; an illuminationsystem configured to condition a radiation beam; a support configured tosupport a patterning device, the patterning device being configured toimpart the radiation beam with a pattern in its cross-section to form apatterned radiation beam; a substrate table configured to hold asubstrate; a projection system configured to project the patternedradiation beam onto a target portion of the substrate; an inletconfigured to provide an H₂ containing gas in at least part of theapparatus; an inlet configured to provide one or more additionalcompounds selected from the group of hydrocarbon compounds and silanecompounds; and a hydrogen radical generator configured to generateradicals from H₂ from the H₂ containing gas.

According to yet a further embodiment, a lithographic apparatus includesan optical element; an illumination system configured to condition aradiation beam; a support configured to support a patterning device, thepatterning device being configured to impart the radiation beam with apattern in its cross-section to form a patterned radiation beam; asubstrate table configured to hold a substrate; a projection systemconfigured to project the patterned radiation beam onto a target portionof the substrate; an inlet configured to provide an H₂ containing gas inat least part of the apparatus; an inlet configured to provide one ormore additional compounds selected from the group of hydrocarboncompounds and silane compounds; a hydrogen radical generator configuredto generate radicals from H₂ from the H₂ containing gas; and adeposition generator, wherein the deposition includes one or moreelements selected from B, C, Si and Ge.

According to yet another embodiment of the present invention, a methodfor the removal of a deposition on an optical element of an apparatusincluding the optical element, and wherein the deposition includes oneor more elements selected from the group of B, Si, Ge and Sn, includesproviding an H₂ containing gas and one or more additional compoundsselected from the group of hydrocarbon compounds and silane compounds inat least part of the apparatus; producing hydrogen radicals from H₂ fromthe H₂ containing gas; and bringing the optical element with depositioninto contact with at least part of the hydrogen radicals and removing atleast part of the deposition.

In yet a further embodiment, the deposition includes at least Si.

According to yet another embodiment of the present invention, anapparatus includes an optical element; and a deposition generatorconfigured to provide a deposition including Sn.

In yet a further embodiment, the apparatus further includes an inletconfigured to providing a starting material including Sn. The startingmaterial may be a gas that can be used to generate a cap layer with CVDtechniques, for example using a hot filament, but the starting materialmay also be a metal or oxide of Sn, which may provide elemental Sn to bedeposited by PVD techniques.

According to another embodiment, the apparatus is a lithographicapparatus for EUV lithography.

According to an embodiment of the invention, the hydrocarbon compoundincludes a C₁-C₂₅ compound. In a further embodiment, the hydrocarboncompound includes a linear, a branched, a cyclic or an aromatichydrocarbon.

In yet another embodiment, the hydrocarbon compound includes an aromaticcompound, substituted with at least one C₁-C₆ hydrocarbon group, whichmay be linear, branched, cyclic or aromatic.

In an embodiment the hydrocarbon compound includes a R₁R₂R₃CR₄ compound,wherein R₁, R₂, R₃ and R₄ independently are selected from the group ofC₁-C₆ hydrocarbon groups, i.e. a carbon atom is coordinated by 4hydrocarbon groups, which may be independently of each other linear,branched, cyclic or aromatic.

In another embodiment, the hydrocarbon compound includes a R₁R₂R₃CR₄compound, wherein R₁, R₂, R₃ and R₄ independently are selected from thegroup of C₁-C₆ hydrocarbon groups and a hydrogen group, and wherein atleast one of R₁, R₂, R₃ and R₄ includes a hydrogen group.

In another embodiment, the hydrocarbon compound is selected from thegroup of methane, toluene and R₁R₂R₃CH, wherein R₁, R₂, and R₃independently are selected from the group of C₁-C₄ hydrocarbon groups,which may be linear, branched or cyclic.

In another embodiment, the silane compound includes a R₁R₂R₃SiR₄compound, wherein R₁, R₂, R₃ and R₄ independently are selected from thegroup of C₁-C₆ hydrocarbon groups, i.e. a silicon atom is coordinated by4 hydrocarbon groups, which may be independently of each other linear,branched, cyclic or aromatic.

In yet another embodiment, the silane compound includes a R₁R₂R₃SiR₄compound, wherein R₁, R₂, R₃ and R₄ independently are selected from thegroup of C₁-C₆ hydrocarbon groups and a hydrogen group, wherein at leastone of R₁, R₂, R₃ and R₄ includes a hydrogen group.

According to yet another embodiment of the invention, the silanecompound includes one or more compounds selected from the group ofSi_(n)H_(2n+2), wherein n is an integer of 1 or larger, e.g. 1(“silane”) or 2, 3, 4 etc. (polysilanes).

Further, in another embodiment, the C₁-C₂₅ compound, the C₁-C₆hydrocarbon side groups or the C₁-C₄ hydrocarbon side groups may includeone or more selected from the group of alkoxy groups, alcohol groups andhalide groups. The C₁-C₆ hydrocarbon side groups or the C₁-C₄hydrocarbon side groups may include linear, branched, cyclic (oraromatic) groups.

In order to provide the hydrocarbon or silane compounds of theinvention, in an embodiment the apparatus of the invention furtherincludes an inlet configured to provide one or more additional compoundsselected from the group of hydrocarbon compounds and silane compounds.In an embodiment, this inlet is the same inlet as configured to providean H₂ containing gas in at least part of the apparatus. In a variant, anumber of inlets is used. In another variant, the H₂ containing gas andthe additional compound (e.g. methane), are mixed before introducinginto at least part of the apparatus.

According to an embodiment, the method according to the inventionfurther includes producing radicals, especially hydrocarbon radicals,from the one or more additional compounds selected from the group ofhydrocarbon compounds and silane compounds.

Further, in another embodiment, the apparatus further includes a radicalgenerator configured to generate radicals from the one or moreadditional compounds. In a specific embodiment, the apparatus furtherincludes a hydrocarbon radical generator configured to generatehydrocarbon radicals from the one or more additional compounds. In afurther specific embodiment, this hydrocarbon radical generator may bethe same generator as used for generation hydrogen radicals, i.e. one ormore radical formation devices selected from a hot filament, a plasma,and radiation; and/or a catalyst for converting the silane intohydrocarbon radicals.

According to an embodiment of the invention, at least part of thedeposition or at least part of the cap layer is removed ex situ from theapparatus including the optical element, is removed in situ from theapparatus including the optical element, or is removed in situ from theapparatus including the optical element while performing an opticalprocess (like a device manufacturing process).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying schematic drawings inwhich corresponding reference symbols indicate corresponding parts, andin which:

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the invention;

FIG. 2 schematically depicts a side view of an EUV illumination systemand projection optics of a lithographic projection apparatus accordingto FIG. 1;

FIG. 3 schematically depicts how a cap layer can be generated accordingto an embodiment of the present invention;

FIG. 4 schematically depicts how a deposition or cap layer can beremoved according to an embodiment of the present invention;

FIG. 5 schematically depicts a part of a collector mirror and filamentsaccording to an embodiment of the present invention;

FIG. 6 schematically depicts a part of a collector mirror withtemperature controller according to an embodiment of the presentinvention;

FIG. 7 schematically depicts an arrangement of a mirror according to anembodiment of the present invention; and

FIG. 8 schematically depicts the transmission of H₂, CH₄, SiH₄ and Argasses relative to the pressure.

FIG. 9 schematically depicts the removal of a Sn layer by addingdifferent amounts of methane to hydrogen gas.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus 1 according to anembodiment of the present invention. The apparatus 1 includes anillumination system (illuminator) IL configured to condition a radiationbeam B (e.g. UV radiation or EUV radiation). A support (e.g. a masktable) MT is configured to support a patterning device (e.g. a mask) MAand is connected to a first positioning device PM configured toaccurately position the patterning device in accordance with certainparameters. A substrate table (e.g. a wafer table) WT is configured tohold a substrate (e.g. a resist-coated wafer) W and is connected to asecond positioning device PW configured to accurately position thesubstrate in accordance with certain parameters. A projection system(e.g. a refractive projection lens system) PL is configured to projectthe patterned radiation beam B onto a target portion C (e.g. includingone or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support MT supports, for example bears the weight of, the patterningdevice MA. It holds the patterning device in a manner that depends onthe orientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support can usemechanical, vacuum, electrostatic or other clamping techniques to holdthe patterning device. The support may be a frame or a table, forexample, which may be fixed or movable as required. The support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system.

Any use of the terms “reticle” or “mask” herein may be consideredsynonymous with the more general term “patterning device.”

The term “patterning device” as used herein should be broadlyinterpreted as referring to any device that can be used to impart aradiation beam with a pattern in its cross-section such as to create apattern in a target portion of the substrate. It should be noted thatthe pattern imparted to the radiation beam may not exactly correspond tothe desired pattern in the target portion of the substrate, for exampleif the pattern includes phase-shifting features or so called assistfeatures. Generally, the pattern imparted to the radiation beam willcorrespond to a particular functional layer in a device being created inthe target portion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” as used herein should be broadlyinterpreted as encompassing any type of projection system, includingrefractive, reflective, catadioptric, magnetic, electromagnetic andelectrostatic optical systems, or any combination thereof, asappropriate for the exposure radiation being used, or for other factorssuch as the use of an immersion liquid or the use of a vacuum. Any useof the term “projection lens” herein may be considered as synonymouswith the more general term “projection system”.

As here depicted, the apparatus is of a reflective type, for exampleemploying a reflective mask. Alternatively, the apparatus may be of atransmissive type, for example employing a transmissive mask.

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather that liquid is located, for example, between the projectionsystem and the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives radiation from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation is passed from the source SO tothe illuminator IL with the aid of a delivery system BD including, forexample, suitable directing mirrors and/or a beam expander. In othercases the source may be an integral part of the lithographic apparatus,for example when the source is a mercury lamp. The source SO and theilluminator IL, together with the beam delivery system BD if required,may be referred to as a radiation system.

The illuminator IL may include an adjusting device AD configured toadjust the angular intensity distribution of the radiation beam.Generally, at least the outer and/or inner radial extent (commonlyreferred to as σ-outer and σ-inner, respectively) of the intensitydistribution in a pupil plane of the illuminator can be adjusted. Inaddition, the illuminator IL may include various other components, suchas an integrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support (e.g., mask table MT), and ispatterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PL, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioning device PW and a position sensor IF2 (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioning device PM and a position sensor IF1(e.g. an interferometric device, linear encoder or capacitive sensor)can be used to accurately position the mask MA with respect to the pathof the radiation beam B, e.g. after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioning device PM. Similarly, movement of the substrate tableWT may be realized using a long-stroke module and a short-stroke module,which form part of the second positioning device PW. In the case of astepper, as opposed to a scanner, the mask table MT may be connected toa short-stroke actuator only, or may be fixed. Mask MA and substrate Wmay be aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions. These are known as scribe-lane alignment marks.Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

In the context of the present invention, “optical element” includes oneor more elements selected from optical filters, optical gratings,mirrors such as multilayer mirrors, grazing incidence mirrors, normalincidence mirrors, collector mirrors, etc, lenses, reticles, diodes,sensors such as intensity measurements sensors, energy sensors, CCDsensors, alignments sensors such as optical alignment sensors, gasbarrier structures, for example as described in U.S. Pat. Nos. 6,614,505and 6,359,969, which are incorporated herein by reference, etc. Suchoptical elements, such as filters, gratings, mirrors or lenses may beflat or curved and may be present as layer, foil, device etc. In anembodiment of the present invention, optical elements such as opticalfilters, optical gratings, mirrors, like multilayer mirrors, grazingincidence mirrors, normal incidence mirrors, collector mirrors, lenses,etc., may be blazed or optimized for radiation with a predeterminedwavelength λ (like 5-20 nm, i.e. EUV radiation, 248 nm, 193 nm, 157 nmor 126 nm, etc). They may also be transmissive to radiation withwavelengths λ in the case of lenses, or reflective in the case ofmirrors, or diffractive in the case of gratings. Some optical elementsmay provide one or more of these optical effects, see for exampleEuropean Patent Applications 03077155 and 03077850, which areincorporated herein by reference.

The term “layer” as used herein, as understood by those of ordinaryskill in the art, may describe layers having one or more boundarysurfaces with other layers and/or with other media such as vacuum inuse. However, it should be understood that “layer” may also mean part ofa structure. The term “layer” may also indicate a number of layers.These layers can be, for example, next to each other or on top of eachother, etc. They may also include one material or a combination ofmaterials. It should also be noted that the term “layers” used hereinmay describe continuous or discontinuous layers. In the presentinvention, the term “material” used herein may also be interpreted as acombination of materials. The term “deposition” herein refers tomaterial that is chemically or physically attached to a surface (e.g.the surface of an optical element), as known to those of ordinary skillin the art. Such deposition may be a layer, but it may also include amulti-layer structure. The deposition may include a cap layer but it mayalso include undesired deposits like sputtered (elemental) particlesfrom a source. The deposition may also include redeposition products orevaporation products. The deposition may also include a cap layer asprotection layer including such sputtered particles, for example afteruse of an apparatus with a source that sputters particles, or includinga deposition from material including one of more elements selected fromthe group of B, C, Si, Ge and Sn. The term “element” in the phrase“wherein the deposition includes one or more elements selected from B,C, Si, Ge and Sn,” or “wherein the cap layer includes one ore moreelements selected from B, C, Si, and Ge,” herein refers to a depositionor cap layer including these elements, or including particles includingsuch elements, or including compounds such as Si oxides, Si carbides,etc., including these elements, or combinations thereof, as will beknown to one of ordinary skill in the art. The phrase “depositionincluding one or more elements selected from B, C, Si and Ge,” may referto mono- or multi-layers including atomic B, C, Si or Ge, orcombinations thereof.

The phrase “a starting material including one or more elements selectedfrom B, C, Si and Ge” refers to gasses (e.g. hydrides), compounds (e.g.Si oxide), metals and elements of B, C, Si and Ge. The phrase “a gasincluding one or more elements selected from B, C, Si and Ge,” refers tothese elements in the gas phase and/or to volatile compounds of B, C, Siand Ge, for example hydrides, although also other volatile compoundsmight be chosen such as metal-hydrocarbon compounds, for example tetramethyl silane, tetra ethyl silane, etc., as will be understood by thoseof ordinary skill in the art. The starting material (or the gas) can beused to provide the species that are able to generate a deposition. Forexample, SiH₄ gas may attach to the surface of an optical element andgenerate, by heat of a hot filament or by radiation etc., a Si caplayer. Herein, the species is the starting material itself. Further,e.g. SiH₄ may de converted to species like SiH₂ and Si, that may form aSi cap layer or deposition on the optical element. Herein the speciesare SiH₂ and Si. In another example, the starting material is e.g. SiOor SiO₂ and by a PVD process Si elements are provided that form adeposition on the optical element. Here the species are Si elements.Hence, the phrase “species that are able to generate a deposition”refers to the starting materials or intermediate products or elementsthat are able to generate the deposition or cap layer, and the term“starting material” refers to elements, gases, metals, metal oxides etc,that may provide by e.g. a chemical vapor deposition process or aphysical vapor deposition process the species that generate thedeposition or cap layer on the optical element by these processes. Aswill be understood by those of ordinary skill in the art, as there maybe some oxygen left in an apparatus for EUV application, notwithstandingthe presence of gasses like Ar, etc., the deposition or cap layer on theoptical element may also include oxides. Further, due to the presence ofhydrocarbons, carbides or oxycarbides may also be formed.

The phrase “providing one or more additional compounds selected from thegroup of hydrocarbon compounds and silane compounds in at least part ofthe apparatus” describes the deliberate introduction of a hydrocarboncompound or a silane compound or both these compounds. Here, “compound”refers to a gas or a liquid, not to one specific molecule. In case of aliquid, in an embodiment, the compound is introduced in the apparatus asgaseous compound, e.g. by a (pre)heating, or is vaporized within theapparatus, e.g. lithographic apparatus, or is introduced afterdecomposition in the apparatus or is decomposed within the apparatus. Inembodiment, the boiling temperature of the hydrocarbon or silanecompound is below 773 K; in a further embodiment the boiling temperatureof the hydrocarbon or silane compound is below 623 K; in yet a furtherembodiment the boiling temperature of the hydrocarbon or silane compoundis below 523 K; in yet another further embodiment the boilingtemperature of the hydrocarbon or silane compound is below 423 K (150°C.). The term “group” or “side chain” or “side group” are synonyms. Forexample, trimethyl silane (HSi(CH₃)₃) is a silicon including onehydrogen group (H) and three methyl groups, or e.g. iso-butane includespropane with 1 methyl side group or side chain. The term C1-C6, etc.,refer to hydrocarbons or hydrocarbon side chains, like C₁ (i.e. CH₄compound or CH₃ side chain), C₂ (ethane), C₃ (propane), C₄ (butane), C₅(pentane), C₆, (hexane), etc., and their isomers.

The terms “radiation” and “beam” as used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength λ of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUV or soft X-ray) radiation (e.g. having a wavelength inthe range of 5-20 nm), as well as particle beams, such as ion beams orelectron beams. Generally, radiation having wavelengths between about780-3000 nm (or larger) is considered IR radiation. UV refers toradiation with wavelengths of approximately 100-400 nm. Withinlithography, it is usually also applied to the wavelengths which can beproduced by a mercury discharge lamp: G-line 436 nm; H-line 405 nm;and/or I-line 365 nm. VUV is Vacuum UV (i.e. UV absorbed by air) andrefers to wavelengths of approximately 100-200 nm. DUV is Deep UV, andis usually used in lithography for the wavelengths produced by excimerlasers like 126 nm-248 nm. As will be understood by those of ordinaryskill in the art, radiation having a wavelength in the range of e.g.5-20 nm relates to radiation with a certain wavelength band with, ofwhich at least part of it is found in the range of 5-20 nm.

FIG. 2 shows the projection apparatus 1 in more detail, including aradiation system 42, an illumination optics unit 44, and the projectionsystem PL. The radiation system 42 includes the radiation source SOwhich may be formed by a discharge plasma. EUV radiation may be producedby a gas or vapor, such as Xe gas, Li vapor or Sn vapor in which a veryhot plasma is created to emit radiation in the EUV range of theelectromagnetic spectrum. The very hot plasma is created by causing apartially ionized plasma of an electrical discharge to collapse onto anoptical axis O. Partial pressures of 10 Pa of Xe, Li, Sn vapor or anyother suitable gas or vapor may be required for efficient generation ofthe radiation. The radiation emitted by radiation source SO is passedfrom a source chamber 47 into a collector chamber 48 via a gas barrierstructure or contamination trap 49 which is positioned in or behind anopening in source chamber 47. The gas barrier structure/contaminationtrap 49 includes a channel structure such as, for example, described indetail in U.S. Pat. Nos. 6,614,505 and 6,359,969.

The collector chamber 48 includes a radiation collector 50 which may beformed by a grazing incidence collector. Radiation passed by collector50 is reflected off a grating spectral filter 51 to be focused in avirtual source point 52 at an aperture in the collector chamber 48. Fromcollector chamber 48, a radiation beam 56 is reflected in illuminationoptics unit 44 via normal incidence reflectors 53, 54 onto a reticle ormask positioned on reticle or mask table MT. A patterned beam 57 isformed which is imaged in projection system PL via reflective elements58, 59 onto wafer stage or substrate table WT. More elements than shownmay generally be present in illumination optics unit 44 and projectionsystem PL.

The radiation collector 50 may be a collector as described in EuropeanPatent Application 03077675.1, which is incorporated herein byreference.

All optical elements shown in FIG. 2, and optical elements not shown inthe schematic drawing of this embodiment, are vulnerable to depositionof, for example, Sn. This is especially the case for optical elementsthe radiation collector 50 and the grating spectral filter 51. Hence,the method of the present invention can be applied to those opticalelements, but also to normal incidence reflectors 53, 54 and reflectiveelements 58, 59. FIG. 3 schematically depicts how a cap layer can begenerated to such optical elements according to an embodiment of thepresent invention.

FIG. 3 schematically depicts a part of the surface of an optical element100 and is intended to schematically describe the process of providing acap layer 125 to the optical element 100 by a deposition process. Itshould be understood that optical element 100 can be a collector 50, agrating spectral filter 51, it can be a multi-layer mirror such as aSi/Mo multi-layer mirror, with for example a Ru coating, etc.

A gas 105, that may provide species that can generate the formation of acap layer 125 (in this embodiment SiH₄ has been taken as example of agaseous starting material) is introduced by a gas supply 6(1) through aninlet 2(1). The deposition of Si is realized due to the presence of aspecies providing device, in this embodiment a hot filament 110, thatprovides a species that can be deposited, which produces amongst othersspecies Si 115, which forms deposition or cap layer 125, and furtherhydrogen 120. The hot filament 110 may be a W (tungsten) or Ta(tantalum) wire or coil, which can be heated to e.g. 1500-3000 K or evenabove, depending upon the kind of material and desired temperature andrequired temperature to dissociate the chemical bonds of the species ingas 105, such that cap layer 125 can be formed due to the deposition ofSi 115. For example, about 20 A can be sent through a thin wire of Ta.The filament reaches a typical temperature of around 2000-2300 K (for Wtypically about 1700-2500 K), and this temperature is high enough tobreak the Si—H bond or C—H bond of hydrocarbons or silanes, producinghydrogen radicals. Si particles (especially atoms) 115 are formed andwill deposit on the optical element 100 as deposition or cap layer 125.Hydrogen radicals or hydrogen gas 120 is also formed.

SiH₄ is a volatile compound. The partial pressure of SiH₄, and otherparameters like gas flow of gas 105, temperature of optical element 100,temperature of gas 105, temperature of filament 110, etc., may beselected and controlled such that a desired deposition rate is obtained.

Instead of SiH₄, gas 105 may also include other silanes or Si-compoundsthat form Si upon heating with a hot filament 110, but gas 105 may alsoinclude hydrocarbons, like CH₄, ethane, propane, (iso-)butane, etc., orcombinations of two or more of these, or combinations of Si and Ccompounds that are able to form deposits. In this way Si can bedeposited with typical growth rates of up to about 30-60 nm/min. C canbe grown with growth rates of up to about 30-60 nm/min. Layerthicknesses of cap layer 125 of Si, C or both Si and C can be betweenabout 0.5 and 20 nm, for example between about 1 and 5 nm. The cap layerhas a roughness between about 1-50% of the layer thickness, for examplebetween about 10 and about 20% of the layer thickness. Instead of ornext to Si and/or C, also B and/or Ge can be selected as material to bedeposited on optical element 100.

Filament 110 may be placed at a distance of about 0.1 to 50 cm fromoptical element 100, depending upon the kind of optical element, thetemperature that is applied and the local pressure in the apparatus atthe place of filament 110 and optical element 100. It should beappreciated that filament 110 may include a number of filaments,arranged at appropriate distances of the surface of optical element 100.It should also be appreciated that filament 110 (or filaments) may beheated (individually) in a controlled way by power supplies (not shown)and further controlled by thermo-elements or other devices (also notshown) that may optically measure the temperature of the optical element100, or the local temperature of a part of optical element 100 or of thetemperature of the space, or of part of the space between opticalelement 100 and filament 110. It should also be understood that layer125 is deposited in time and thus FIG. 3 schematically shows a certainmoment in time where such layer 125 is present.

During at least part of the use of the apparatus, for example alithographic apparatus, Sn may deposit on at least part cap layer 125 ordamage and remove at least part of cap layer 125, due to emission of thesource SO (not shown). After use of optical element 100, cap layer 125may appear to be damaged and may include elements such as Sn, which arenot desired or which may potential harm optical element 100, andtherefore cap layer 125 may be removed and replaced by a fresh cap layer125. FIG. 4 also schematically depicts a part of the surface of opticalelement 100 and is intended to schematically describe the process ofremoving a deposition or cap layer 125. Layer 125 may be the cap layeras described above in FIG. 3, it may also be such cap layer 125 afteruse of an apparatus including optical element 100 with cap layer 125,wherein, due to the use the cap layer, next to one or more elementsselected from B, C, Si and Ge, cap layer 125 also includes Sn, forexample, from source SO (shown in FIGS. 1 and 2), or layer 125 maymainly or entirely include undesired deposition such as Sn orhydrocarbons from plastic tubes or rest gasses, etc.

Referring to FIG. 4, a gas 155 including H₂ is provided by gas supply6(2) through inlet 2(2) in the space where optical element 100 ispresent. Due to the presence of hot filament 110, hydrogen gasdissociates in hydrogen radicals 165. Part of the hydrogen radicals 165will come into contact with deposition or cap layer 125 on the surfaceof optical element 100, where the hydrogen radicals 165 may react withone or more of Si (including Si oxides), Sn (including tin oxides) andC, etc. In this way, optical element 100 with deposition 125 is broughtinto contact with at least part of the hydrogen radicals 165 and atleast part of the deposition 125 is removed. Volatile hydrides 170 areformed, which in FIG. 4 are indicated as SiH₄, but which can alsoinclude tin hydrides and/or hydrocarbons such as CH₄, etc. Volatilecompounds 170 may be at least partially removed by an exhaust or pump(not shown in FIGS. 3 and 4). However, one or more getter plates mayalso be provided, on which the hydrides may form deposits, which arethen not detrimental to the optical elements anymore. Removal rates of100-150 nm/hour may be obtained. Growth and removal rates may be derivedfrom ex situ X-ray fluorescence spectroscopy. In this way, a method isused wherein cap layer 125 is provided to optical element 100 by adeposition process; and wherein during or after use of the apparatus atleast part of cap layer 125 is removed from optical element 100 in aremoval process. Optical element 100 is protected during use, and maycontinuously be used, due to a refreshment of cap layer 125. After sometime of use, optical element 100 with cap layer 125 may be subjected toabove mentioned removal process including providing an H₂ containing gas155 in at least part of the apparatus; producing hydrogen radicals 165from H₂ from the H₂ containing gas; and bringing optical element 100with the cap layer 125 into contact with at least part of the hydrogenradicals 165 and removing at least part of cap layer 125. Without theuse of halides, as suggested in the prior art, which may damage parts ofthe apparatus including the optical element 100, the deposition or caplayer 125 may at least be partially be removed, thereby removing damagedcap layer 125 or a cap layer 125 including Sn. Even Sn that may haveoxidized may be removed by hydrogen radicals 165 as Sn hydride. Thesevolatile compounds may be removed from the apparatus by an exhaust (notshown). By performing the method of the present invention, opticalelement 100 is protected from Sn by cap layer 125, leading to betteroptical properties and a longer lifetime of optical element 100. Forexample, reflection loss due to Sn can be diminished, and protective caplayer 125 can easily be renewed for prolonged life time of opticalelement 100. Next to that, cap layer 125, including one or more selectedfrom B, C, Si and Ge, is relatively transmissive for EUV radiation, suchthat radiation is substantially not diminished by cap layer 125.

In this embodiment, the device that produces hydrogen radicals is afilament 110, and the device that provides deposition from one or moreselected from B, C, Si and Ge is also a filament 110. These filamentscan be the same or can be different filaments 110.

Gas supplies 6(1) and 6(2) and inlets 2(1) and 2(2) may be the same gassupplies and inlets, respectively. Further, these inlets can be normalinlets that may be used to introduce gasses in the apparatus or toremove gasses from the apparatus. Hence, these supplies and inlets, aswill be appreciated by those of ordinary skill in the art, representthose devices that enable introduction and, where applicable, enableremoval of gasses from the apparatus or parts of the apparatus whichinclude optical element 100. The devices may contain pumps, vessels,(partial) pressure means, flow controllers, valves, exhausts, etc., asshould be appreciated by those of ordinary skill in the art.

The cap layer 125 does not only protect optical element 100 againstsputtering products or electrons from a source, it also protects opticalelement 100 against chemical attacks of, for example, oxygen or water.

In a variation of this embodiment, FIG. 4 may refer to an opticalelement 100 which includes a mirror 58 or 59 (see FIG. 2), or anothermirror or optical element 100 that may be present in projection systemPL (in this system PL, 6 mirrors may be present, for example). Opticalelements present in the projection system PL and also other parts of theapparatus may include deposition 125, formed by outgassing of Si fromthe resist on the wafer on waver table WT (see FIGS. 1 and 2). Accordingto the method depicted in FIG. 4, this deposition may be removed withhydrogen radicals 165, generated by hot filament 110 or by other devicesdescribed herein. Further, gas supply 6(2) may provide hydrogen gas 155,but may further provide halogens. Hydrogen radicals may reduce oxides ofSi present in deposition 125 to elemental Si, which may further beremoved by producing volatile hydrides and halides, respectively. Incontrast to methods known in the art using halides only, with the methodof the present invention, oxides of, for example, Si and Sn may also beremoved.

Referring to FIG. 3, gas 105 including CH₄ is introduced by supply 6(1)via inlet 2(1). The deposition of C is realized due to the presence of ahot filament 110, which may be heated to 1500-3000 K, or even above todissociate the chemical bonds of CH₄ in gas 105, such that cap layer 125can be formed due to the deposition of C 115. Hydrogen radicals orhydrogen gas 120 is also formed. Instead of CH₄, other hydrocarbons mayalso be used.

After use of optical element 100, cap layer 125 may be removed andreplaced by a fresh cap layer 125. Referring to FIG. 4, a gas 155including H₂ is provided by supply 6(2) via inlet 2(2) to the spacewhere optical element 100 is present. Due to the presence of hotfilament 110, hydrogen gas dissociates in hydrogen radicals 165. Part ofthe hydrogen radicals 165 will migrate to the deposition or cap layer125 on the surface of optical element 100, where the hydrogen radicals165 may react with C or C and Sn (sputtered from the source SO), etc.Volatile hydrides 170 are formed, which in FIG. 4 are indicated as SiH₄,but which in this embodiment include hydrocarbons or hydrocarbons andtin hydrides. Volatile compounds 170 may be at least partially removedby an exhaust or pump (not shown in FIG. 3 or 4).

In an embodiment of the present invention, optical element 100 may be aradiation collector 50 (see also FIG. 2). FIG. 5 schematically depicts apart of a collector 50, including a first mirror 30(1) and a secondmirror 30(2). Radiation collector 50 may include more than two mirrorsto collect EUV radiation 35 and to a produce radiation beam 56. Withinspace 180 between mirrors 30(1) and 30(2), there are some parts of space180 which are arranged outside the EUV radiation 35, i.e., in the shadowof mirror 30(2). These parts of space are indicated by gray areas. Forexample, these spaces can be used to arrange a filament 110, asindicated in the figure with filaments 110(1) and 110(2). Hydrides ofcarbon or silicon may enter space 180 and by heating filaments 110, caplayers (not shown) are formed on mirrors 30(1) and 30(2). Likewise,hydrogen gas may enter space 180 and may be at least partially convertedby filaments 110 to hydrogen radicals 165, which may remove a undesireddeposition such as Sn or a cap layer 125 such as a carbon cap layer, asilicon cap layer or a carbon-silicon cap layer etc., wherein theselatter layers may also include Sn from source SO (not shown).

In a variation on this embodiment, gas supplies 6(1) and 6(2) may bepositioned in the space where collector 50 is positioned. In a furthervariation, gas supplies may be positioned in such a way, that inlets2(1) and 2(2) are also in the parts of space 180 which are arrangedoutside the EUV radiation 35, i.e., in the shadow of mirror 30(2). FIG.5 shows examples of positions of the gas supplies 6(1) and 6(2),respectively. It should be appreciated that one or more gas supplies6(1) and 6(2), respectively, may be positioned in the space wherecollector 50 (or other mirrors) are present, but only one gas supply 6with one inlet 2 (not shown) may alternatingly provide the desiredgasses.

In yet a further embodiment of the present invention, the opticalelement 100 may be a collector 50, which is further provided with aheating element 38. FIG. 6 schematically depicts a part of a collector50, including a first mirror of 30(1) and a second mirror 30(2). Theradiation collector 50 may include more than two mirrors to collect EUVradiation 35 and to produce radiation beam 56. Within space 180 betweenmirrors 30(1) and 30(2), there are some parts of space 180 which arearranged outside the EUV radiation 35, i.e., in the shadow of mirror30(2). These parts of space are indicated by gray areas with referencenumbers 32 and 34. For example, these spaces can be used to arrange afilament 110 (as shown in FIG. 5). FIG. 6 schematically shows a heatingelement 38 controlled by a controller 40. The heating element source 38may be connected to mirror 30(1) by connectors 31. The connectors 31 maybe heated conductively. The controller 40 may be implemented as asuitably programmed computer, or a controller with suitable analogand/or digital circuits. The heating element 38 generates heat,indicated with arrows 37, which is directed to the first mirror of 30(1)of radiation collector 50. Heating element 38 may also be physicallyconnected to mirror 30(1). Heater 38 may include different heatingelements and/or may be arranged such that different areas of mirror30(1) can selectively be heated. In this way, layer growth and layerremoval of a cap layer may better be controlled. Heating element 38 maybe controlled by controller 40, which may also control a pump 29, ormeasuring devices such as thermo-couples, gas pressure, gas flow, ananalysis unit that analyzes reflectivity of mirror 30(1), an analysisunit that measures cap layer thickness, etc (not shown in FIG. 6, butsome of these devices are shown in FIG. 7).

In yet a further embodiment of the present invention, as schematicallyshown in FIG. 7, optical element may be a radiation collector 50, whichmay be a multilayer mirror, which is further provided with a heatingelement 68. Such multilayer mirror may include at least 40 layers, or atleast 50 layers, at least 60 layers, and may further include a Ruprotection layer (top layer).

The system as depicted in FIG. 7 includes a source chamber 47 whichradiates radiation into a chamber 3, which may be a part of collectorchamber 48 (shown in FIG. 2) or may be collector chamber 48 itself. Thesystem includes a gas supply 6 to supply a hydrocarbon gas, a silanegas, or both. The supply includes a source of the required gas 6 and aninlet 2 (in this figure, only one gas supply 6 is shown, but moresupplies may be present, see for example FIGS. 3-5). Gas is supplied tochamber 3 and the partial pressure of the gases in chamber 3 may bemonitored via the sensor 5 (which may be coupled to a controller, forexample controller 40 in FIG. 5). A reflectivity sensor 7 may beprovided and may be used to determine when the reflectance of collector50 decreases or increases, due to sputtering of Sn, growth of Si, and/orC, etc., layers and removal of deposition or cap layers 125,respectively. Further, outlet 22 may be provided to remove hydrogen gas,hydrides and where applicable halides. Although this embodimentdescribes multilayer mirrors, it is not intended to limit the presentinvention. One alternative is a grazing incidence mirror, mirrors ontowhich radiation is directed at an angle smaller than about 20°, which donot necessarily include multilayers, but can be manufactured of a singlemetal layer. Such mirrors degrade as well when exposed to the atoms andions generated by the EUV radiation source. The same methods can also beapplied to these mirrors.

Outlet 22 may be used as exhaust for removing, for example, H₂,hydrocarbons, etc., but also for removing, for example, volatile (by)products formed in the methods of the invention, such as H₂O, hydridesand halides of Sn, Si, B, C, Ge and hydrocarbons which may be formed,such as C₂H₄, etc. Such outlets may also be present at other places, forexample close to optical element 100 in FIGS. 3, 4, 5 and 6.

An application of the method for providing a cap layer 125 will bedescribed with reference to FIGS. 2-4. Using a chemical vapor depositiontechnique such as hot-filament deposition, silicon or carbon can bedeposited on and etched from optical element 100. This gives a thinlayer 125 of Si or C (or a layer of Si and C), etc., as cap layer on theoptic element(s), which will protect it from etching by fast ions comingfrom the source SO. The layer 125 may be dynamic, because cap layer 125may continuously be etched and deposited in order to keep a cap layer ofa certain thickness and roughness. This can be done in three separateprocesses, namely (1) deposit layer 125, then (2) operate lithographicapparatus, and (3) stop apparatus and etch away layer 125 (which may bediminished and damaged by the fast atoms and ions such as Sn from thesource SO and which may also include Sn, next to C, Si or both Si andC). Alternatively, in yet another embodiment, all three processes can bedone simultaneously, using online etching and deposition duringlithography operation. Materials that can be used as dynamic cap layer125 are Si, C, B and Ge, or combinations of two or more of theseelements can be used. Etching and depositing of the silicon (or B, C,Ge) layer can be done online, i.e. when the EUV lithography machine isrunning. This means that while irradiating the optical element 100 withthe radiation beam at least part of cap layer 125 is provided to opticalelement 100 or that while irradiating optical element 100 with theradiation beam at least part of cap layer 125 is removed from opticalelement 100, respectively. This might be possible if the pressures ofthe gasses can be high enough. Hydrogen has a very good transmission forEUV radiation, which means that online etching may also be possible.Fortunately, SiH₄ (a source for Si deposition) and CH₄ (a source for C)have a very good transmission for EUV radiation and have even a bettertransmission than argon, which is a widely used buffer gas at themoment, see FIG. 8, wherein the transmission of those gasses, relativeto the pressure, is given. When deliberately introducing these gasses asgas 105 or 155, respectively, the pressure difference between thechambers or closed part of a chamber (e.g. radiation system 42 in FIG. 2or chamber 3 in FIG. 7) with connecting chambers or connecting otherparts of the chambers, respectively, needs not be very large. Further,SiH₄ and CH₄ may serve as buffer gasses for EUV debris suppression aswell.

Different spots on optical element 100 may experience different etchingrates due to the fast ions from the source and deposition rates. Thismakes it difficult to achieve equilibrium with a uniform cap layerthickness over the entire surface of the optical element 100. This mightbe solved with temperature gradients over the optical element 100 byheating elements (see for example FIGS. 6 and 7), in order to increasethe etch rates in certain parts of optical element 100. Cooling elementsmay also be used. Hence, a part or parts of optical element 100 may beheated while providing cap layer 125 to optical element 100 or whileremoving cap layer (or deposition) 125 and a part or parts of opticalelement 100 may be cooled while providing cap layer 125 to opticalelement 100, such that a uniform cap layer 125 can be provided and caplayer 125 can also uniformly be removed, at least partially. Cooling maybe performed with peltier elements; heating may be performed withheating elements and also with IR radiation. Other ways to control thethickness and uniformity of cap layer 125 may be by varying thepressures, of the gas, the temperature of filament 110, distance offilament 110 to optical element 100, or individual temperatures anddistances of different filaments 110, etc.

In another embodiment, cap layer 125 is generated by physical vapordeposition (PVD). To this end, elemental B, C, Si or Ge, or combinationsof one or more of these elements is introduced in an apparatus includingoptical element 100 in such a way that a cap layer 125 is formed onoptical element 100. For example, a beam of electrons hits a startingmaterial including B, C, Si or Ge, which leads to a formation of speciesthat are able to generate a deposition by sputtering and/or evaporationof elements and/or particles of B, C, Si or Ge, respectively. Physicalvapor deposition may be performed in situ of a lithographic apparatus,thereby providing an on line deposition technique, or ex situ in a PVDapparatus, thereby providing an off line technique.

In this embodiment, the deposition generator generates a depositionselected from one or more B, C, Si and Ge and is a source of radiation,for example a source providing an electron bombardment. After or duringuse of the optical element, cap layer 125 may be removed by using ahydrogen radical generator, such as a filament that can be heated, aplasma generator, a source of radiation, which may be the same source,and a catalyst to convert H₂ into hydrogen radicals.

In another embodiment, referring to FIG. 4, next to hydrogen 155,halogens may also be introduced. For example Sn, which may have beenpartly oxidized by the presence of oxygen, can be reduced with hydrogenradicals 165, produced by hot filament 110. Sn may than further beremoved as SnH₄ 170 but also as Sn halide. For example, next tohydrogen, gas 155 may also include Cl₂ or I₂. Less halogen has to beused than in currently known processes. The same applies for a Si or Gecap layer or deposition.

In yet another embodiment, instead of filament 110, a plasma generatoris used to generate hydrogen radicals 165. This generator may be a highvoltage generator, for example about 5-10 kV, between two electrodes,thereby forming a plasma. Due to the presence of this plasma, hydrogenfrom gas 155, including hydrogen, is at least partially converted tohydrogen radicals 165.

Such plasma may also be used to generate Si from gas 105 includingsilanes as starting material or C from gas 105 including hydrocarbons asstarting material, respectively. In this embodiment, the hydrogenradical generator is a plasma, and the deposition generator to generatea deposition from one or more of B, C, Si and Ge may be filament 110,the plasma or a source of radiation.

In yet another embodiment, next to filament 110, a catalyst to convertH₂ into hydrogen radicals is used. This may be a Ru containing catalystsuch as zeolite including Ru or alumina with Ru, or other metals such asRh, Ir, Pt, Pd, etc. Due to the presence of the catalyst to convert H₂into hydrogen radicals, hydrogen from gas 155, including hydrogen, areat least partially converted to hydrogen radicals 165. In a variation onthis embodiment, the catalyst and optical element 100 include a Rulayer, which may be a Ru protection layer on optical element 100 such asa mirror, for example a multilayer mirror, grazing incidence mirror,etc. In this embodiment, hydrogen radical generator is a catalyst, andthe deposition generator may be filament 110, or another device toprovide cap layer 125.

In the context of this embodiment, especially in the context of a Rulayer on an optical element, hydrogen gas molecules may be dissociatedon the surface of Ru, leading to hydrogen atoms being chemisorbed to thesurface. The hydrogen atoms may migrate to e.g. Sn and remove Sn ashydride or migrate to Sn oxides (e.g. SnO/SnO₂) and reduce them to Sn.Hydrogen atoms may also be formed on the surface in close proximity ofSn or Sn oxides (or Si and SiO/SiO₂, respectively, etc.) and remove Snor reduce SnO, respectively. Elemental Sn may then be removed byhydrogen radicals, hydrogen atoms bound to the surface of the Ru layerand by halides (see also e.g. embodiments 1 and 8).

In yet another embodiment, a cap layer 125 is generated by introducinghydrocarbons to the apparatus and providing hydrocarbons in theproximity of optical element 100. Due to irradiation with a source, thehydrocarbons which are attached (e.g. due to Van der Waals forces) tothe surface of optical element 100 are at least partially converted to acap layer 125 including C. The source may provide EUV, DUV or UVradiation. Alternatively or additionally, a source may be used thatprovides electrons, which may also convert hydrocarbons to C, such thata cap layer 125 including C is provided. In a variation on thisembodiment, e.g. the source of radiation may be source SO (see FIGS. 1and 2) and/or a separate source of radiation.

In this embodiment of the present invention, optical element 100including deposition or cap layer 125 is removed from the apparatus thatincludes this optical element 100, and the deposition or cap layerremoval process is performed ex situ in another setup by providing an H₂containing gas in at least part of this setup; producing hydrogenradicals from H₂ from the H₂ containing gas; and bringing the opticalelement 100 with the cap layer 125 into contact with at least part ofthe hydrogen radicals 165 and removing at least part of the cap layer125 from optical element 100 in this setup.

In this embodiment of the present invention, optical element 100 isremoved from the apparatus that includes this optical element 100, andthe deposition to generate cap layer 125 is performed ex situ in anothersetup by a deposition generator, wherein the deposition includes one ormore elements selected from B, C, Si and Ge. Alternatively, also a thinlayer of Sn may be deposited, for example a few nm, such as about 1-50nm, or about 2-40 nm.

In this embodiment, cap layer 125 is removed after a certain period ofuse in an apparatus. The removal is performed to a certain referencevalue (e.g. reflection of optical element 100, in case optical 100 is amirror or grating), to be detected by sensor 7 of FIG. 7. This processcan be repeated after each use of the apparatus or after some time ofuse of the apparatus. The cap layer 125 does not necessarily need to becompletely removed, but only needs to be removed until a certainreference value is obtained. After having removed part of cap layer 125,fresh cap layer 125 can be generated, until a second reference value isobtained. In this way, a fresh uniform layer can be obtained, eitherafter a complete removal of cap layer 125 or after a partial removal ofcap layer 125.

In this embodiment, the apparatus is a lithographic apparatus forlithography and the beam of radiation includes radiation. Opticalelement 100 is an optical element, for example a diode to senseradiation intensity, or a diode to sense optical properties of otheroptical elements 100, or a sensor 7 as in FIG. 7 to measure thereflectivity of a mirror, or a sensor for alignment, etc. Thelithographic apparatus may be designed to irradiate the optical elementwith a beam of radiation, for example in the case of a sensor to sensethe radiation intensity, but the sensors mentioned above may also bepresent in the apparatus at position where no direct radiation ispresent. A cap layer 125 may be generated that is substantiallytransparent for radiation, for example a few nm of Si.

According to another embodiment, the apparatus includes a lithographicapparatus, for example the apparatus according to FIG. 2, designed forEUV lithography.

Referring to FIGS. 4-7, for a removal process a hydrocarbon and/orsilane compound is provided by supply 6(2) through inlet 2(2) in thespace where optical element 100 is present (in addition to hydrogengas), i.e. reference symbol 155 may also indicate a mixture or two ormore flows of a hydrogen containing gas and a hydrocarbon and/or silanecompound gas (or gas containing hydrocarbon and/or silane). Supply 6 mayrefer to a number of supplies, for example a supply for the hydrocarboncompound, a supply for a silane compound (used for removing depositionor cap layer 125), a supply for a hydrogen containing gas and a supplyfor silane (for providing cap layer 125). For example, for removingdeposition or cap layer 125, a hydrocarbon compound and a hydrogencontaining gas may be provided to the apparatus (e.g. H₂/CH₄). Thesegasses may be introduced separately or may be mixed before introducingto the apparatus.

For example, by adding CH₄ to the hydrogen containing gas duringcleaning, the cleaning rate is advantageously increased relative to acleaning process under the same conditions but with only hydrogen. Whene.g. CH₄ is added to the hydrogen gas, in an embodiment the hot filament110 will dissociate CH₄ molecules in addition to the H₂ molecules (i.e.at least part of the gasses will produce hydrocarbon and hydrogenradicals; and hydrogen radical generator 110 is also a hydrocarbonradical generator 110). The resulting radicals will be H radicals ande.g. CH₃ radicals. The ratio between these radicals can be tuned by e.g.changing the relative gas flows of hydrogen containing gas and thehydrocarbon compound containing gas CH₄ or by tuning the molecularratios in the mixed gas. The person skilled in the art will tune theratio such that the ratio is not too low or too high. For example, ifthere are too many CH₃ radicals, there is a high risk of forming a thicklayer of carbon on the substrate (in addition to a desired carbonizingeffect). Therefore the CH₃ concentration needs to be low enough to allowthe H radicals to remove the carbon deposits faster than they aregrowing. However, if the CH₃ concentration is too low, the effect of theCH₄ addition will be too low.

The mechanism with which the hydrocarbon radicals can improve thecleaning rate may be one or more of the following:

In the gas-phase the hydrocarbon radicals, e.g. CH₃ radicals, can reactwith the SnH₄ (tin-hydride) molecules to form SnH₃(CH₃), SnH₂(CH₃)2,SnH(CH₃)₃, or Sn(CH₃)₄. These molecules are more stable than the SnH₄molecule and therefore the cleaning rate will improve, since thesemolecules have a lower chance of forming an Sn re-depositing on themirror. Hence, referring to e.g. FIG. 4, reference symbol 170 mayinclude SiH₄, but may further include e.g. one or more of SnH₃(CH₃),SnH₂(CH₃)2, SnH(CH₃)₃, or Sn(CH₃)₄ (assuming that the additionalcompound forms methyl radicals in the removal method of the invention);

The hydrocarbon radicals, e.g. CH₃ radicals, can carbonize the top layerof the mirror. For example, this can effectively change e.g. a Rusubstrate into a Ru-carbide substrate. It may be that the Ru-carbidesubstrate has a better Sn cleaning rate than a pure Ru substrate.

This may similarly apply for other depositions, especially B, C, Si andGe.

In an embodiment of the invention, the hydrocarbon compound includes aC₁-C₂₅ compound. In a further embodiment, the hydrocarbon compoundincludes a linear, a branched, a cyclic or an aromatic hydrocarbon, e.g.methane, ethane, n-propane, n-butane, n-propane, n-hexane, and isomersthereof like cyclo propane, cyclo butane, tert butane (iso-butane or2-methyl propane), cyclo pentane, iso-pentane (or 2-methylbutane),neo-pentane (or 2,2-dimethylpropane) etc. In yet a further embodiment,the hydrocarbon compound is selected from methane, ethane, n-propane,n-butane, n-propane, n-hexane, cyclo butane, tert butane (iso-butane),iso-pentane, neo-pentane, and mixtures of one or more of thesecompounds. Further, the hydrocarbon compound may have one or moreunsaturated bonds, i.e. an alkene, alkyne or aromatic. Combinations ofone or more of these hydrocarbon compounds may also be used.

In an embodiment the hydrocarbon compound includes a R₁R₂R₃CR₄ compound,wherein R₁, R₂, R₃ and R₄ (side chains) independently are selected fromthe group of C₁-C₆ hydrocarbon groups, i.e. a carbon atom is coordinatedby 4 hydrocarbon groups, which may be independently of each otherlinear, branched, cyclic or aromatic, e.g. in an embodiment C(CH₃)₄,C(CH₂CH₃)₄, C(CH₂CH₂CH₃)₄, C(CH₂CH₂CH₂CH₃)₄, etc. but also in anembodiment e.g. C(CH₃)(CH₂CH₂CH₃)₃, etc. The side chains may alsoinclude in an embodiment isomers of these hydrocarbon groups, forexample those defined above. The side chains may also includeunsaturated hydrocarbon groups. The side chains may also be different(independently chosen), and may be linear, branched or cyclic.

In a further embodiment, the hydrocarbon compound includes a R₁R₂R₃CR₄compound, wherein R₁, R₂, R₃ and R₄ independently are selected from thegroup of C₁-C₆ hydrocarbon groups and a hydrogen group, and wherein atleast one of R₁, R₂, R₃ and R₄ includes a hydrogen group. In a specificembodiment, the hydrocarbon compound is selected from HCR₁R₂R₃, whereinR₁, R₂, and R₃ independently are selected from the group of C₁-C₆hydrocarbon groups, like CH(CH₃)₃ (trimethylmethane), CH(CH₂CH₃)₃(triethylmethane), CH(CH₂CH₂CH₃)₃ (tripropylmethane), CH(CH₂CH₂CH₂CH₃)₃(tributylmethane), CH(CH₂CH₂CH₂CH₂CH₃)₃ (tripentylmethane) andCH(CH₂CH₂CH₂CH₂CH₂CH₃)₃ (trihexylmethane). In yet another specificembodiment, the hydrocarbon compound is selected from the group ofmethane, toluene and R₁R₂R₃CH, wherein R₁, R₂, and R₃ independently areselected from the group of C₁-C₄ hydrocarbon groups, which may belinear, branched or cyclic, like e.g. CH(CH₃)₃ (trimethylmethane),CH(CH₂CH₃)₃ (triethylmethane), CH(CH₂CH₂CH₃)₃ (tripropylmethane),CH(CH₂CH₂CH₂CH₃)₃ (tributylmethane). Like above, the side chains mayalso include in an embodiment isomers of these hydrocarbon groups, forexample those defined above. The side chains may also includeunsaturated hydrocarbon groups. The side chains may also be different,and may be linear, branched or cyclic.

In yet another embodiment, the hydrocarbon compound includes an aromaticcompound, substituted with at least one C₁-C₆ hydrocarbon group, whichmay be linear, branched, cyclic or aromatic.

Hence, hydrocarbon compounds may be used in both the deposition process(cap layer 125 formation) and removal process (i.e. removal of cap layeror deposition 125).

Instead of a hydrocarbon compound or in addition to the hydrocarboncompound, also a silane compound may be used. Both compounds may providea function similar to a catalyst in the sense that the removal rate ofthe deposition or cap layer by treatment with hydrogen radicals isincreased. In an embodiment, the silane compound includes a R₁R₂R₃SiR₄compound, wherein R₁, R₂, R₃ and R₄ independently are selected from thegroup of C₁-C₆ hydrocarbon groups, i.e. a silicon atom is coordinated by4 hydrocarbon groups, which may be independently of each other linear,branched, cyclic or aromatic, like e.g. in an embodiment Si(CH₃)₄,Si(CH₂CH₃)₄, Si(CH₂CH₂CH₃)₄, Si(CH₂CH₂CH₂CH₃)₄, etc., but also in anembodiment Si(CH₃)(CH₂CH₂CH₃)₃, etc. Like above, the side chains mayalso include in an embodiment isomers of these hydrocarbon groups, forexample those defined above. The side chains may also includeunsaturated hydrocarbon groups. The side chains may also be different,and may be linear, branched or cyclic.

In another embodiment, the silane compound includes a R₁R₂R₃SiR₄compound, wherein R₁, R₂, R₃ and R₄ independently are selected from thegroup of C₁-C₆ hydrocarbon groups and a hydrogen group, wherein at leastone of R₁, R₂, R₃ and R₄ includes hydrogen, i.e. a silicon atom iscoordinated by 3 or less hydrocarbon groups, which may be independentlyof each other linear, branched, cyclic or aromatic (e.g. tri or dimethyl silane). The hydrocarbon side chains may have one or moreunsaturated bonds, i.e. an alkene, alkyne or aromatic. In a specificembodiment, the hydrocarbon compound is selected from HSiR₁R₂R₃, whereinR₁, R₂, and R₃ independently are selected from the group of C₁-C₆hydrocarbon groups, like SiH(CH₃)₃ (trimethylsilane), SiH(CH₂CH₃)₃(triethylsilane), SiH(CH₂CH₂CH₃)₃ (tripropylsilane), SiH(CH₂CH₂CH₂CH₃)₃(tributylsilane), SiH(CH₂CH₂CH₂CH₂CH₃)₃ (tripentylsilane) andSiH(CH₂CH₂CH₂CH₂CH₂CH₃)₃ (trihexylsilane). In yet another specificembodiment, the silane compound is selected from the group of R₁R₂R₃CH,wherein R₁, R₂, and R₃ independently are selected from the group ofC₁-C₄ hydrocarbon groups, which may be linear, branched or cyclic, likee.g. SiH(CH₃)₄ (trimethylsilane), SiH(CH₂CH₃)₄ (triethylsilane),SiH(CH₂CH₂CH₃)₄ (tripropylsilane), SiH(CH₂CH₂CH₂CH₃)₄ (tributylsilane).Like above, the side chains may also include in an embodiment isomers ofthese hydrocarbon groups, for example those defined above. The sidechains may also include unsaturated hydrocarbon groups. The side chainsmay also be different, and may be linear, branched or cyclic. In anotherembodiment, the silane includes a polysilane.

Further, in a specific embodiment, the C₁-C₂₅ compound, the C₁-C₆hydrocarbon side groups or the C₁-C₄ hydrocarbon side groups for eitherthe hydrocarbon or the silane compound may include one or more selectedfrom the group of alkoxy groups (e.g. alkylalkoxysilane), alcohol groupsand halide groups. As will be clear to the person skilled in the art,combinations of gasses can be used

For all above mentioned side chains apply that in an embodiment the sidechains may include isomers of these hydrocarbon groups, for examplethose defined above. The side chains may also include unsaturatedhydrocarbon groups. The side chains may also be different (within onecompound, e.g. ethyl dimethyl silane: (CH₃CH₂)SiH(CH₃)₂, etc.) and maybe linear, branched or cyclic.

In yet another embodiment, the silane compound includes one or morecompounds selected from the group of Si_(n)H_(2n+2), wherein n is aninteger of 1 or larger. Hence, silanes, in a specific embodimentSi_(n)H_(2n+2) silanes, may be used both in the deposition process (caplayer 125 formation) and removal process.

In order to provide the hydrocarbon or silane compounds of theinvention, in an embodiment the apparatus of the invention furtherincludes an inlet 6, 6(2) configured to provide one or more additionalcompounds selected from the group of hydrocarbon compounds and silanecompounds. In an embodiment, this inlet 6,6(2) is the same inlet 6,6(2)as configured to provide an H₂ containing gas in at least part of theapparatus.

According to an embodiment, the method according to the inventionfurther includes producing radicals from the one or more additionalcompounds selected from the group of compounds and silane compounds. Forexample, CH₃ may be formed from CH₄, tert butyl methane or from tertmethyl silane (CH₃)₃SiH, etc. According to a specific embodiment, themethod according to the invention further includes producing hydrocarbonradicals from the one or more additional compounds selected from thegroup of hydrocarbon compounds and silane compounds. To this end, in aspecific embodiment, the apparatus further includes a hydrocarbonradical generator configured to generate hydrocarbon radicals from theone or more additional compounds. In a further specific embodiment, thishydrocarbon radical generator may be the same generator as used forgeneration hydrogen radicals, i.e. one or more radical formation devicesselected from a hot filament, a plasma, radiation; and/or a catalyst forconverting the silane into hydrocarbon radicals. For example, referringto FIGS. 4 and 5, filaments 110 may be used to generate hydrogenradicals 165 and hydrocarbon radicals.

Likewise, in a specific embodiment, the apparatus further includes aradical generator configured to generate radicals from the one or morethe silane compounds selected from the group of Si_(n)H_(2n+2), whereinn is an integer of 1 or larger. In a further specific embodiment, thisradical generator may be the same generator as used for the generationof hydrogen radicals or of hydrocarbon radicals, as described above,i.e. one or more radical formation devices selected from a hot filament(reference symbol 110 in FIGS. 4 and 6), a plasma, radiation; and/or acatalyst for converting the silane compound into radicals.

According to an embodiment of the invention, at least part of thedeposition 125 or at least part of the cap layer 125 is removed ex situfrom the apparatus including the optical element. This means thatoptical element 100 is removed from the optical apparatus and broughtinto another apparatus or vessel for removing at least part ofdeposition 125. According to another embodiment of the invention, atleast part of the deposition 125 or at least part of the cap layer 125is removed in situ from the apparatus including the optical element. Forexample, a space containing the optical element is provides with abovementioned hydrogen containing gas an a hydrocarbon and/or silanecompounds and by a radical generator, deposition 125 is removed.According to yet another embodiment of the invention, at least part ofthe deposition 125 or at least part of the cap layer 125 is removed insitu from the apparatus including the optical element while performingan optical process (like a device manufacturing method). For example,while using a lithographic apparatus, hydrogen gas and methane gas maybe provided to a collector mirror 50 module, like radiation system 42and by a hot filament 110 or a plasma, hydrogen and hydrocarbon radicalsare formed, removing advantageously at least part of the undesireddeposition 125 or of e.g. a degenerated cap layer 125. In situ (or exsitu) also a new, fresh or improved cap 125 may be provided as describedabove.

In this embodiment, cap layer or deposition 125 is removed after acertain period of use in an apparatus. The removal is performed to acertain reference value (e.g. reflection of optical element 100, in caseoptical 100 is a mirror or grating), to be detected by sensor 7 of FIG.7. This process can be repeated after each use of the apparatus or aftersome time of use of the apparatus. The cap layer 125 does notnecessarily need to be completely removed, but only needs to be removeduntil a certain reference value is obtained. After having removed partor all of cap layer 125, fresh cap layer 125 can be generated, until asecond reference value is obtained. In this way, a fresh uniform layercan be obtained, either after a complete removal of cap layer 125 orafter a partial removal of cap layer 125. By monitoring e.g. reflection,or other parameters, the person skilled in the art can tune formation ofcap layer 125 and removal of deposition 125. Hence, by monitoring such aparameter, also the amount and where applicable ratio of components tobe introduced for removal can be tuned, and also the amount and whereapplicable the ratio of components to be introduced for cap layerformation can be tuned. For example, assuming removal by using a gasincluding a hydrocarbon compound, e.g. methane gas, and hydrogen,depending upon the parameter that is monitored, the amount(s), gasflow(s), temperature, pressure, etc. can be tuned such that the desiredamount of deposition 125 is removed.

As shown in FIG. 9, the removal process is enhanced by the presence of ahydrocarbon relative to a pure hydrogen process. By adding a few percentmethane (1, 3, 5 and 8 vol. %, relative to hydrogen) the same number oftreatments results is a considerable decrease in Sn thickness. In theexperiment of FIG. 9, there is no substantial further decrease in Snthickness if the amount of methane is increased from 5 to 8%; the curvessubstantially overlap each other.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. It should be appreciated that, in the context of suchalternative applications, any use of the terms “wafer” or “die” hereinmay be considered as synonymous with the more general terms “substrate”or “target portion”, respectively. The substrate referred to herein maybe processed, before or after exposure, in for example a track (a toolthat typically applies a layer of resist to a substrate and develops theexposed resist), a metrology tool and/or an inspection tool. Whereapplicable, the disclosure herein may be applied to such and othersubstrate processing tools. Further, the substrate may be processed morethan once, for example in order to create a multi-layer IC, so that theterm substrate used herein may also refer to a substrate that alreadycontains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

The invention is not limited to application of the lithographicapparatus or use in the lithographic apparatus as described in theembodiments. Further, the drawings usually only include the elements andfeatures that are necessary to understand the invention. Beyond that,the drawings of the lithographic apparatus are schematically and not onscale. The invention is not limited to those elements, shown in theschematic drawings (e.g. the number of mirrors drawn in the schematicdrawings). Further, the invention is not confined to the lithographicapparatus described in FIGS. 1 and 2. The person skilled in the art willunderstand that embodiments described above may be combined. Further,the invention is not limited to protection against, for example Sn froma source SO, but also other particles from other sources, such asradiation sources that may damage optical element 100, may be removedwith the methods of the present invention. The methods for protection ofan optical element may also be applied likewise with respect to suchother particles. The embodiments that mention, for example only C or Si,may also be directed to B, Ge and Sn. Further, cap layer 125 may begenerated by a deposition process providing a cap layer 125 includingother elements than B, C, Si and Ge, when cap layers 125 including suchelements can at least partially be removed by hydrogen radicals.

The scope of protection of the invention is not limited to theembodiments given. The invention resides in each novel characteristicand each combination of characteristics.

1. A method for the removal of a deposition on an optical element of anapparatus including the optical element, the method comprising:providing an H₂ containing gas and one or more additional compoundsselected from the group of hydrocarbon compounds and silane compounds inat least part of the apparatus; producing hydrogen radicals from H₂ fromthe H₂ containing gas; producing radicals from the one or moreadditional compounds; and removing at least part of a protectivedeposition from an optical element by having the optical element be incontact with at least part of the hydrogen radicals and the radicalsproduced from the one or more additional compounds.
 2. The methodaccording to claim 1, wherein the deposition comprises one or moreelements selected from the group of B, C, Si, Ge and Sn.
 3. The methodaccording to claim 1, wherein at least part of the hydrogen radicals aregenerated from H₂ from the H₂ containing gas by a filament, a plasma,radiation, or a catalyst configured to convert H₂ into hydrogenradicals.
 4. The method according to claim 1, wherein the H₂ containinggas further comprises a halogen gas.
 5. The method according to claim 1,wherein the apparatus is a lithographic apparatus.
 6. The methodaccording to claim 1, wherein the optical element comprises an opticalelement selected from the group of a mirror, a grating, a reticle and asensor.
 7. The method according to claim 1, wherein the at least part ofthe deposition is removed ex situ from the apparatus including theoptical element.
 8. The method according to claim 1, wherein thehydrocarbon compound comprises a C₁-C₂₅ compound.
 9. The methodaccording to claim 1, wherein the hydrocarbon compound comprises alinear, a branched, a cyclic or an aromatic hydrocarbon.
 10. The methodaccording to claim 1, wherein the hydrocarbon compound comprises aR₁R₂R₃CR₄ compound, wherein R₁, R₂, R₃ and R₄ independently are selectedfrom the group of C₁-C₆ hydrocarbon groups.
 11. The method according toclaim 1, wherein the hydrocarbon compound comprises a R₁R₂R₃CR₄compound, wherein R₁, R₂, R₃ and R₄ independently are selected from thegroup of C₁-C₆ hydrocarbon groups and a hydrogen group, and wherein atleast one of R₁, R₂, R₃ and R₄ comprises a hydrogen group.
 12. Themethod according to claim 1, wherein the hydrocarbon compound comprisesan aromatic compound, substituted with at least one C₁-C₆ hydrocarbongroup.
 13. The method according to claim 1, wherein the hydrocarboncompound is selected from the group of methane, toluene and R₁R₂R₃CH,wherein R₁, R₂, R₃ and R₄ independently are selected from the group ofC₁-C₄ hydrocarbon groups.
 14. The method according to claim 1, whereinthe silane compound comprises a R₁R₂R₃SiR₄ compound, wherein R₁, R₂, R₃and R₄ independently are selected from the group of C₁-C₆ hydrocarbongroups.
 15. The method according to claim 1, wherein the silane compoundcomprises a R₁R₂R₃SiR₄ compound, wherein R₁, R₂, R₃ and R₄ independentlyare selected from the group of C₁-C₆ hydrocarbon groups and a hydrogengroup, and wherein at least one of R₁, R₂, R₃ and R₄ comprises ahydrogen group.
 16. The method according to claim 1, wherein the silanecompound comprises a HSiR₁R₂R₃ compound, wherein R₁, R₂, and R₃independently are selected from the group of C₁-C₆ hydrocarbon groups.17. The method according to claim 1, wherein the silane compoundcomprises one or more compounds selected from the group ofSi_(n)H_(2n+2), wherein n is an integer of 1 or larger.
 18. A method forthe protection of an optical element of an apparatus including theoptical element, the method comprising: providing a cap layer to theoptical element by a deposition process; and during or after use of theapparatus, removing at least part of the cap layer from the opticalelement in a removal process comprising providing an H₂ containing gasand one or more additional compounds selected from the group ofhydrocarbon compounds and silane compounds in at least part of theapparatus; producing hydrogen radicals from H₂ from the H₂ containinggas; producing radicals from the one or more additional compounds; andremoving at least part of the cap layer from the optical element byhaving the optical element be in contact with at least part of thehydrogen radicals and the radicals produced from the one or moreadditional compounds.
 19. The method according to claim 18, wherein thedeposition process comprises: providing a starting material comprisingone or more elements selected from B, C, Si and Ge; providing, from thestarting material, a species comprising one or more elements selectedfrom B, C, Si and Ge that is able to generate a deposition; and bringingthe species that is able to generate a deposition into contact with atleast part of the optical element.
 20. The method according to claim 18,wherein the species that is able to generate a deposition is generatedby a filament, a plasma or radiation.
 21. The method according to claim18, wherein the cap layer comprises one or more elements selected fromthe group of B, C, Si and Ge.
 22. The method according to claim 18,wherein the use of the apparatus comprises irradiating the opticalelement with a radiation beam.
 23. The method according to claim 18,wherein at least part of the cap layer is generated ex situ from theapparatus including the optical element.
 24. The method according toclaim 18, wherein the at least part of the cap layer is removed ex situfrom the apparatus including the optical element.
 25. The methodaccording to claim 18, wherein the hydrocarbon compound comprises aC₁-C₂₅ compound.
 26. The method according to claim 18, wherein thehydrocarbon compound comprises a linear, a branched, a cyclic or anaromatic hydrocarbon.
 27. The method according to claim 18, wherein thehydrocarbon compound comprises a R₁R₂R₃CR₄ compound, wherein R₁, R₂, R₃and R₄ independently are selected from the group of C₁-C₆ hydrocarbongroups.
 28. The method according to claim 18, wherein the hydrocarboncompound comprises a R₁R₂R₃CR₄ compound, wherein R₁, R₂, R₃ and R₄independently are selected from the group of C₁-C₆ hydrocarbon groupsand a hydrogen group, and wherein at least one of R₁, R₂, R₃ and R₄comprises a hydrogen group.
 29. The method according to claim 18,wherein the hydrocarbon compound comprises an aromatic compound,substituted with at least one C₁-C₆ hydrocarbon group.
 30. The methodaccording to claim 18, wherein the hydrocarbon compound is selected fromthe group of methane, toluene and R₁R₂R₃CH, wherein R₁, R₂, R₃ and R₄independently are selected from the group of C₁-C₄ hydrocarbon groups.31. The method according to claim 18, wherein the silane compoundcomprises a R₁R₂R₃SiR₄ compound, wherein R₁, R₂, R₃ and R₄ independentlyare selected from the group of C₁-C₆ hydrocarbon groups.
 32. The methodaccording to claim 18, wherein the silane compound comprises aR₁R₂R₃SiR₄ compound, wherein R₁, R₂, R₃ and R₄ independently areselected from the group of C₁-C₆ hydrocarbon groups and a hydrogengroup, and wherein at least one of R₁, R₂, R₃ and R₄ comprises ahydrogen group.
 33. The method according to claim 18, wherein the silanecompound comprises a HSiR₁R₂R₃ compound, wherein R₁, R₂, and R₃independently are selected from the group of C₁-C₆ hydrocarbon groups.34. The method according to claim 18, wherein the silane compoundcomprises one or more compounds selected from the group ofSi_(n)H_(2n+2), wherein n is an integer of 1 or larger.
 35. A devicemanufacturing method, comprising: generating a beam of radiation usingan illumination system of a lithographic apparatus; projecting apatterned beam of radiation onto a target portion of a substrate; andduring or after use of the lithographic apparatus, removing at leastpart of a cap layer from an optical element in a removal processcomprising providing an H₂ containing gas and one or more additionalcompounds selected from the group of hydrocarbon compounds and silanecompounds in at least part of the lithographic apparatus, producinghydrogen radicals from H₂ from the H₂ containing gas, producing radicalsfrom the one or more additional compounds, and removing at least part ofthe cap layer from the optical element by having the optical element bein contact with at least part of the hydrogen radicals and the radicalsproduced from the one or more additional compounds.
 36. An apparatus,comprising: an optical element having a protective deposition; an inletconfigured to provide an H₂ containing gas in at least part of theapparatus; an inlet configured to provide one or more additionalcompounds selected from the group of hydrocarbon compounds and silanecompounds; and a radical generator configured to generate hydrogenradicals from H₂ from the H₂ containing gas and to generate radicalsfrom the one or more additional compounds and to remove at least part ofthe protective deposition from the optical element by having the opticalelement be in contact with at least part of the hydrogen radicals andthe radicals from the one or more additional compounds.
 37. Theapparatus of claim 36, wherein the hydrogen radical generator comprisesone or more devices selected from a filament that can be heated, aplasma generator a source of radiation, and a catalyst configured toconvert H₂ into hydrogen radicals.
 38. The apparatus of claim 36,wherein the apparatus is a lithographic apparatus.
 39. The apparatus ofclaim 36, wherein the optical element comprises an optical elementselected from the group of a mirror, a grating, a reticle and a sensor.40. The apparatus of claim 36, wherein the apparatus, wherein theapparatus further comprises a radical generator configured to generateradicals from the one or more additional compounds.
 41. The apparatus ofclaim 36, wherein the apparatus, wherein the apparatus further comprisesa hydrocarbon radical generator configured to generate hydrocarbonradicals from the one or more additional compounds.
 42. An apparatus,comprising: an optical element; an inlet configured to provide an H₂containing gas in at least part of the apparatus; an inlet configured toprovide one or more additional compounds selected from the group ofhydrocarbon compounds and silane compounds; a deposition generatorconfigured to generate a deposition on the optical element, wherein thedeposition comprises one or more elements selected from B, C, Si and Ge;and a radical generator configured to generate hydrogen radicals from H₂from the H₂ containing gas and to generate radicals from the one or moreadditional compounds and to remove at least part of the protectivedeposition from the optical element by having the optical element be incontact with at least part of the hydrogen radicals and the radicalsfrom the one or more additional compounds.
 43. The apparatus of claim42, wherein the hydrogen radical generator comprises one or more devicesselected from the group of a filament that can be heated, a plasmagenerator, a source of radiation, and a catalyst configured to convertH₂ into hydrogen radicals.
 44. The apparatus of claim 42, wherein thedeposition generator comprises one or more devices selected from thegroup of a filament that can be heated, a plasma generator and a sourceof radiation.
 45. The apparatus of claim 42, further comprising an inletconfigured to provide a starting material comprising one or moreelements selected from the group of B, C, Si and Ge.
 46. The apparatusof claim 42, further comprising an inlet configured to provide a gascomprising one or more elements selected from the group of B, C, Si andGe.
 47. The apparatus of claim 42, wherein the apparatus, wherein theapparatus further comprises a radical generator configured to generateradicals from the one or more additional compounds.
 48. The apparatus ofclaim 42, wherein the apparatus, wherein the apparatus further comprisesa hydrocarbon radical generator configured to generate hydrocarbonradicals from the one or more additional compounds.
 49. A lithographicapparatus, comprising: an optical element having a protectivedeposition; an illumination system configured to condition a radiationbeam; a support configured to support a patterning device, thepatterning device being configured to impart the radiation beam with apattern in its cross-section to form a patterned radiation beam; asubstrate table configured to hold a substrate; a projection systemconfigured to project the patterned radiation beam onto a target portionof the substrate; an inlet configured to provide an H₂ containing gas inat least part of the apparatus; an inlet configured to provide one ormore additional compounds selected from the group of hydrocarboncompounds and silane compounds; and a radical generator configured togenerate hydrogen radicals from H₂ from the H₂ containing gas and togenerate radicals from the one or more additional compounds and toremove at least part of the protective deposition from the opticalelement by having the optical element be in contact with at least partof the hydrogen radicals and the radicals from the one or moreadditional compounds.
 50. A lithographic apparatus, comprising: anoptical element; an illumination system configured to condition aradiation beam; a support configured to support a patterning device, thepatterning device being configured to impart the radiation beam with apattern in its cross-section to form a patterned radiation beam; asubstrate table configured to hold a substrate; a projection systemconfigured to project the patterned radiation beam onto a target portionof the substrate; an inlet configured to provide an H₂ containing gas inat least part of the apparatus; an inlet configured to provide one ormore additional compounds selected from the group of hydrocarboncompounds and silane compounds; a deposition generator configured togenerate a deposition on the optical element, wherein the depositioncomprises one or more elements selected from B, C, Si and Ge; and aradical generator configured to generate hydrogen radicals from H₂ fromthe H₂ containing gas and to generate radicals from the one or moreadditional compounds and to remove at least part of the deposition fromthe optical element by having the optical element be in contact with atleast part of the hydrogen radicals and the radicals from the one ormore additional compounds.
 51. A method for the removal of a depositionon an optical element of an apparatus including the optical element, andwherein the deposition comprises one or more elements selected from B,Si, Ge and Sn, the method comprising: providing an H₂ containing gas andone or more additional compounds selected from the group of hydrocarboncompounds and silane compounds in at least part of the apparatus;producing hydrogen radicals from H₂ from the H₂ containing gas;producing radicals from the one or more additional compounds; andremoving at least part of the deposition from the optical element byhaving the optical element be in contact with at least part of thehydrogen radicals and the radicals produced from the one or moreadditional compounds.
 52. The apparatus of claim 36, further comprisinga heater configured to heat at least a part of the optical element. 53.The apparatus of claim 36, further comprising a cooler configured tocool at least a part of the optical element to establish a temperaturegradient across the optical element during removal of the at least partof the protective deposition.
 54. The apparatus of claim 36, furthercomprising a reflectivity sensor configured to detect reflectance of theoptical element.
 55. The apparatus of claim 54, configured to continueremoving the at least part of the protective deposition until a certainreflectivity value of the optical element measured by the reflectivitysensor is obtained.